The voltage-sensitive sodium channel is being studied in Drosophila melanogaster using the neurotoxin, saxitoxin, which is known to bind specifically to sodium channels and to block the action potential. A rapid filtration assay has been developed to study 3H-saxitoxin binding to homogenates of Drosophila heads. Initial studies have shown that the toxin binds to a membrane associated component with high affinity. This binding is saturable, reversible, and noncooperative, and is inhibited by low concentrations of tetrodotoxin. The effects of pH and monovalent and divalent cations on 3H-saxitoxin binding will be analyzed. Limited enzymatic digestion will be used to define the biochemical nature of the toxin-binding component and procedures will be developed for solubilizing this component with mixed lipid/detergent micelles. The major emphasis of this study will be to develop a genetic strategy which will allow us to identify genes affecting sodium channel structure. The first approach will be to study the effects of temperature on 3H-saxitoxin binding parameters using extracts of temperature-sensitive paralysis mutants which have been shown to have temperature-induced defects in conduction of action potentials. The second genetic approach will be to use the segmental aneuploid technique to screen for a region of the genome which is dose sensitive with respect to 3H-saxitoxin binding. The final genetic approach proposed is to screen for electrophoretic variants in the saxitoxin-binding component. After identifying the regions of the genome which affect sodium channel structure, we will use small deficiencies for this region or regions to screen for lethal and temperature-sensitive lethal mutants with severe effects on sodium channel function. These studies provide a model system for investigating the organization of genes coding for products involved in cell excitability.